db: add index on posts_gdocs.slug column

[marcelgerber]

Noticed that we don't have an index for that column yet, and it seems super sensible to have one 👀

[owidbot]

Quick links (staging server):

Site	Admin	Wizard

Login: ssh owid@staging-site-db-gdocs-slug-index

• Site-screenshots: https://github.com/owid/site-screenshots/compare/db-gdocs-slug-index
• SVG tester: https://github.com/owid/owid-grapher-svgs/compare/db-gdocs-slug-index

SVG tester:

Number of differences (default views): 0 ✅
Number of differences (all views): 0 ✅

Edited: 2024-06-14 13:35:41 UTC
Execution time: 1.25 seconds

[ikesau]

I don't really know the perf tradeoffs between fully indexing all 255 characters versus the 100 character substring but in the unlikely event we have collisions I doubt it'll matter much anyway. 👍